1. Field of the Invention
The present invention relates to a method for producing an electrochemical element and the electrochemical element. In particular, the present invention relates to a method for producing an electrochemical element and the electrochemical element preferably used for a gas sensor for measuring oxide such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O, and inflammable gases such as H.sub.2 CO, and hydrocarbon (CnHm) contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles.
2. Description of the Related Art
Conventionally, those known as the method for measuring NOx in a measurement gas such as combustion gas include a technique in which the NOx-reducing ability of Rh is utilized while using a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia to measure an electromotive force generated between the both electrodes.
However, the sensor as described above suffers the following problem. That is, the electromotive force is greatly changed depending on the change in concentration of oxygen contained in the combustion gas as the measurement gas. Moreover, the change in electromotive force is small with respect to the change in concentration of NOx. For this reason, the conventional sensor tends to suffer influence of noise.
Further, in order to bring out the NOx-reducing ability, it is indispensable to use a reducing gas such as CO. For this reason, the amount of produced CO is generally smaller than the amount of produced NOx under a lean fuel combustion condition in which a large amount of NOx is produced. Therefore, the conventional sensor has a drawback in that it is impossible to perform correct measurement for a combustion gas produced under such a combustion condition.
In order to solve the problems as described above, for example, Japanese Laid-Open Patent Publication No. 8-271476 discloses a NOx sensor comprising pumping electrodes having different NOx-decomposing abilities arranged in a first internal space which communicates with a measurement gas-existing space and in a second internal space which communicates with the first internal space, and a method for measuring the NOx concentration in which the O.sub.2 concentration is adjusted by using a first pumping cell arranged in the first internal space, and NO is decomposed by using a decomposing pumping cell arranged in the second internal space so that the NOx concentration is measured on the basis of a pumping current flowing through the decomposing pump.
Further, Japanese Laid-Open Patent Publication No. 9-113484 discloses a sensor element comprising an auxiliary pumping electrode arranged in a second internal space so that the oxygen concentration in the second internal space is controlled to be constant even when the oxygen concentration is suddenly changed.
In the case of the NOx sensor as described above, the decomposition of NOx theoretically begins at an element temperature of about 600.degree. C. However, the decomposition of NOx actually begins at an element temperature of about 700.degree. C. in the conventional gas sensor. Therefore, a problem arises in that the NOx-decomposing electrode such as an Rh electrode has poor operation performance at a low temperature.
It is considered that such a problem is caused as follows. That is, a part of the Rh electrode forms an alloy together with Pt and Au during the process of producing the NOx sensor, especially during the sintering process for the substrate. For this reason, the catalytic activity, which is originally possessed by Rh, is lowered. As a result, the NOx sensor has the poor operation performance at the low temperature.
In view of the consideration as described above, the present inventors observed a cross section of such a sensor element by using SEM (scanning electron microscope). As a result, it was found that an alloy was formed by Rh together with Pt and Au on the Rh electrode.
The mechanism for forming the alloy will now be explained. At first, Pt, which is a major material for the Pt electrode provided to be paired with the Rh electrode, is partially oxidized in atmospheric air in a region of not less than 1000.degree. C. to form PtO.sub.2. In general, PtO.sub.2 has a high vapor pressure as compared with metallic Pt, and it has a volatile property.
Volatilized PtO.sub.2 diffuses up to the Rh electrode provided in the substrate together with the Pt electrode, and it forms an alloy together with Rh. Especially, when Au, Ir, or various transition metals are added in order to suppress the catalytic activity of the Pt electrode, the volatilization of Pt is accelerated. As a result, the formation of the alloy of Rh, Pt, Au or the like is facilitated thereby. It is assumed that the acceleration of the volatilization of Pt is caused because the oxidation of Pt is accelerated by the addition of Au or the like to Pt.
When the alloy is formed, an extremely large amount of oxygen exists around the Rh electrode. In order to improve the measurement accuracy of the NOx sensor, it is necessary to eliminate the offset component upon the measurement as less as possible. Therefore, it is necessary that the oxygen existing around the Rh electrode should be once removed when the operation of the NOx sensor is started. As a result, a problem arises in that when the NOx sensor is used, it is necessary to wait for the start of actual measurement for a long period of time.